G

50 k

G 1

R

:

INA188

RG

VIN-

VIN+

+

VOUT

REF

Simplified

Form

18

INA188

SBOS632 – SEPTEMBER 2015

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(1)

NC denotes no connection. When using the SPICE model, the simulation does not converge unless a resistor is connected to the RG

pins; use a very large resistor value.

7.3 Feature Description

7.3.1 Inside the INA188

The Functional Block Diagram section provides a detailed diagram for the INA188, including the ESD protection

and radio frequency interference (RFI) filtering. Instrumentation amplifiers are commonly represented in a

simplified form, as shown in Figure 47.

Figure 47. INA Simplified Form

A brief description of the internal operation is as follows:

The differential input voltage applied across RG causes a signal current to flow through the RG resistor and both

RF resistors. The output difference amplifier (A3) removes the common-mode component of the input signal and

refers the output signal to the REF pin.

The equations shown in the Functional Block Diagram section describe the output voltages of A1 and A2.

Understanding the internal node voltages is useful to avoid saturating the device and to ensure proper device

operation.

7.3.2 Setting the Gain

The gain of the INA188 is set by a single external resistor, RG, connected between pins 1 and 8. The value of RG

is selected according to Equation 1:

(1)

Table 1 lists several commonly-used gains and resistor values. The 50-k

Ω term in Equation 1 comes from the

sum of the two internal 25-k

Ω feedback resistors. These on-chip resistors are laser-trimmed to accurate absolute

values. The accuracy and temperature coefficients of these resistors are included in the gain accuracy and drift

specifications of the INA188.

Table 1. Commonly-Used Gains and Resistor Values

DESIRED GAIN

RG (Ω)

NEAREST 1% RG (Ω)

1

NC(1)

NC

2

50k

49.9k

5

12.5k

12.4k

10

5.556k

5.49k

20

2.632k

2.61k

50

1.02k

1.02k

100

505.1

511

200

251.3

249

500

100.2

100

1000

50.05

49.9